Clock switching circuit with priority multiplexer

ABSTRACT

A glitch free clock switching circuit includes a first enable synchronization logic that generates a first clock enable in response to a first enable from a first enable generation logic. The clock switching circuit includes a second enable synchronization logic that generates a second clock enable in response to a second enable from a second enable generation logic. A logic gate is coupled to an output of the second enable synchronization logic that selects the second clock signal as a logic gate output if the second enable is logic high. A priority multiplexer receives a first clock signal, the first enable and the logic gate output. The multiplexer configured to select the first clock signal as the clock output if the first enable is logic high, irrespective of the logic gate output.

TECHNICAL FIELD

Embodiments of the disclosure relate to clock switching circuits inapplication specific integrated circuits (ASIC).

BACKGROUND

An ideal clock switching circuit switches between a set of clock sourceswithout generating glitches. Inputs to clock switching circuit includetwo asynchronous clocks and a signal which selects one of the clocksources. Output of the clock switching circuit is one of the twoasynchronous clock sources depending on the polarity of the signal thatselects one of the clock sources. During and after power up (when RESETis released) of the clock switching circuit, glitches can occur at theoutput of the clock switching circuit. Say CLK 1 and CLK 2 are the twoasynchronous signals and CLK 1 is the default clock signal. CLK2 may ormay not be active during the power up. If CLK 2 is inactive during powerup, then the flip flop that drives an enable of CLK 2 is not known. Ifthis comes up as logic high and CLK 2 is also at logic high, then theoutput of the clock switching circuit will always be logic high. Thisissue can be solved by using asynchronous reset to the CLK 2 domain flipflops. Since reset is used asynchronously in the CLK2 domain, when resetis released, a recovery or removal violation can occur on the flip flopwhich drives the enable of CLK2-AND gate. If the flip flop is settled to‘1’ then there can be glitch on OR gate output since both clock to ORgate are enabled for 1 or 2 cycles.

Once a clock source has been selected, during the course of operationthe selected clock source can be missing or can become faulty. Incertain applications, which needs to be fault tolerant, there will becircuitry which can monitor the clocks and indicate the faultycondition. Upon the detection of clock fault, this needs to be rectifiedby switching to alternate clock source. This can cause problems as thesynchronizers of the selected clock source cannot change state due toabsence of a clock signal. As a result, the clock switching circuit willnever switch to alternate clock source. Basically a deadlock occurs inthe circuit.

SUMMARY

An example embodiment provides a clock switching circuit. The circuitincludes a first enable synchronization logic that generates a firstclock enable in response to a first enable from a first enablegeneration logic, the first enable synchronization logic configured toreceive a first clock signal. A second enable synchronization logicgenerates a second clock enable in response to a second enable from asecond enable generation logic. The second enable synchronization logicis configured to receive a second clock signal. A logic gate receivesthe second enable and the second clock signal, the logic gate selectsthe second clock signal as a logic gate output if the second enable islogic high. A priority multiplexer receives the first clock signal, thefirst enable and the logic gate output. The multiplexer is configured toselect the first clock signal as the clock output if the first enable islogic high, irrespective of the logic gate output. In another exampleembodiment the clock switching circuit includes a missing clockdetection circuit that detects absence of the second clock signal,wherein if the second clock signal is missing, the missing clockdetection circuit sends a CLK OFF signal to the second enablesynchronization logic that breaks a feedback path such that an output ofthe clock switching circuit is switched from the second clock signal tothe first clock signal in response to the CLK OFF signal. Other aspectsand example embodiments are provided in the Drawings and the DetailedDescription that follows.

BRIEF DESCRIPTION OF THE VIEWS OF DRAWINGS

FIG. 1 illustrates a clock switching circuit;

FIG. 2 illustrates a clock switching circuit according to an embodiment;and

FIG. 3 illustrates a clock switching circuit according to anotherembodiment;

DETAILED DESCRIPTION OF THE EMBODIMENTS

The clock switching circuit as illustrated in FIG. 1 includes two clockdomains, each having enable generation logics (105 and 110), enablesynchronization logics (115 and 120), two AND gates (125 and 130) and anOR gate 135. The enable generation logic 105 receives a CLK select, CLK2enable and a RESET CLK2. Similarly, enable generation logic 110 receivesa CLK select, CLK1 enable and RESET CLK2. Outputs of the enablegeneration logics 105 and 110 are connected to enable synchronizationlogics 115 and 120 respectively. The enable synchronization logics (115and 120) include two negative edged flip-flops 115A and 115B and 120Aand 120B connected to each other. Flip-flops receive two asynchronousclock signals CLK1 and CLK2 on lines 145 and 150 respectively. Outputsof the enable synchronization logics, CLK1 enable on line 155 and CLK2enable on line 160, are connected to AND gates 125 and 130 respectively.Outputs of the AND gates are connected to the OR gate 135. An outputfrom the OR gate is the clock output CLK OUT, taken on line 140.

A select signal CLK select (165) is used to select one of the clocksources. In one example, when CLK select (165) is low CLK2 is to beselected and when CLK select (165) is high CLK1 is to be selected. Inorder to make the output of clock switching circuit CLKOUT (140) glitchfree at any point of time, only one clock needs to be enabled by ANDgates 125 and 130.The AND gate 125 outputs CLK1 (145) when CLK1 Enableis high and otherwise it is inactive and the output of AND gate (125) islow. The AND gate 130 will output CLK2 (150) when CLK2 Enable is highotherwise it is turned off and output of AND gate (130) is low. In orderto avoid glitches on AND gate (125), CLK1 Enable (155) should bemodified such that it is to be made high or low on falling transition ofCLK1 (145). Similarly CLK2 Enable should be modified such that it ishigh or low on negative transition of CLK2 otherwise glitches on ANDgate (130) is unavoidable.

To be able to modify the enables on falling transition of clock, thesignal CLK1 Enable (155) and CLK2 Enable (160) are generated by thenegative edge flop which is part of enable synchronization circuits 115and 120 respectively. The D input of synchronization circuits 115 and120 are asynchronous to clock signals CLK1 (145) and CLK2 (150)respectively. Since D input is asynchronous, the D input needs to besynchronized to its clock domain. D input of enable synchronizationlogic (115) is synchronized and synchronized signal called CLK1 Enable(on line 155) is used to activate or inactivate CLK1 on AND gate (125).Similarly D input of enable synchronization logic (120) is synchronizedand synchronized signal called CLK2 Enable (online 160) is used toactivate or inactivate CLK2 on AND gate (130). The front end circuits ofenable synchronizer circuits 115 and 120 are designed such that D inputsof enable synchronizer circuits are not high at any point of time. Inorder to make the D inputs mutually exclusive, enable synchronizationlogic 115 outputs is feedback to enable synchronization logic 120through the enable generation logic 110. Similarly, enablesynchronization logic 120 output is feedback to enable synchronizationlogic 115 through the enable generation logic 105.

As long as CLK2 Enable is high, D input of enable synchronization logic115 is made low irrespective of CLK select (select signal). Similarly aslong as CLK1 Enable is high, D input of enable synchronization logic 120is made low irrespective of CLK select. This circuit has limitationswhen one of the CLK sources CLK1 (145) and CLK2 (150) are not guaranteedto be active always.

It is noted that any clock switch switching circuit will have defaultclock source selection when circuit is powered up. In order to explainthe limitations it is assumed that CLK1 (145) is default source whichmeans clock switching circuit will output CLK1 (145) after reset. Resetsused in this circuit are RESET CLK1 and RESET CLK2. Resets aresynchronous to CLK1 (145) and CLK2 (150) respectively. Resets aregenerated from common RESET. All resets are assumed to active high thatwhen high circuit will be in reset.

When reset is released in the aforementioned clock switching circuit aglitch occurs. This is explained now considering two cases. Case 1: CLK2is not toggling or clocking during reset phase of the design; that issignals RESET CLK1 and RESET CLK2 are high. Since clock to enablesynchronization logic 120 is inactive, D input which is low is notpropagated through enable synchronization logic 120. This makes the CLK2Enable un-initialized. After power up flip flops (120A, 120B) in enablesynchronization logic 120 can get initialized to any value either low orhigh. When CLK2 is faulty it can hold either low or high values. If flipflop 120B in the enable synchronization logic 120 is initialized to highduring power up then CLK2 enable will be high. Assuming CLK2 is holdinga high the output of AND gate (130) is high, which forces output of ORgate (135) to high independent of the other input which is connected toAND gate (125). In such case CLK1 will be not selected as clock sourceand output (140) remains high. This issue can be solved by usingasynchronous reset to flops in enable synchronization logic 120, howeverthis cannot be done in the case when one of the clock signals is faultyfor which the details are explained in case 2 below.

Case 2: Using asynchronous reset to flops 120A and 120B in enablesynchronization logic 120 solves the problem of initialization if CLK2(150) is inactive during reset, assuming that RESET CLK is connected toR (reset pin of the flip flop) of CLK2 synchronizer. If CLK2 (150) isactive, then when RESET CLK which is unsynchronized is released a timingviolation of asynchronous pins of flip flops occurs. In this case,output of the enable synchronization logic 120 can be high for 1 or 2cycles if flops 120A and 120E are resolved to high after meta-stability.Due to this AND gate (130) outputs CLK2 (150) for 1 or 2 cycles, at thesame time AND gate (125) outputs CLK1 (145) as CLK1 is default clocksource. On the input of OR gate (120) both the clocks will be active, asresults of this there will be glitches on CLKOUT (121).

Further, when selected clock source is faulty, switching to the otherclock source can generate glitch at the CLKOUT 140. To enable glitchfree clock switching there includes a feedback path from one clockdomain to the other. The feedback signals are CLK1 Enable (155) and CLK2enable (160) respectively. For instance clock switching circuit isoutputting CLK2 (150) and CLK2 (150) becomes faulty, in this case iftrying to switch to CLK1 (145) by changing CLK select (165) to high willnot switch clocks because front end circuit of enable synchronizationlogic 115 depends on CLK2 Enable (160) and this needs to be low to makethe D input of enable synchronization logic 115 to high. Since CLK2 isfaulty or missing enable synchronization logic 115 will not respondhence CLK2 enable will never become low, as a result of this there isdead lock.

The limitations of clock switching circuit of FIG. 1 are overcome byseveral embodiments as illustrated in FIG. 2 and FIG. 3. Specifically,FIG. 2 illustrates a clock switching circuit that generates a glitchfree clock when reset is released according to an embodiment. Further,FIG. 3 illustrates a clock switching circuit that can switch between theclock sources when a selected clock source is missing or faultyaccording to another embodiment.

Referring now to FIG. 2, the clock switching circuit includes two clockdomains, each having enable generation logics (CLK1 enable generationlogic 205 (first enable generation logic) and CLK2 enable generationlogic 225(second enable generation logic)), enable synchronizationlogics (CLK1 enable synchronization logic 210 (first enablesynchronization logic) and CLK2 enable synchronization logic 230 (secondenable synchronization logic)). The enable generation logic 205 receivesa CLK select, CLK2 enable (forming a second feedback path from secondclock domain to the first clock domain) and a RESET CLK2. Similarly,enable generation logic 230 receives a CLK select, CLK1 enable (forminga first feedback path from first clock domain to the second clockdomain) and

RESET CLK2. Outputs of the enable generation logics 205 and 225 areconnected to enable synchronization logics 210 and 230 respectively.Each of the enable synchronization logics (205 and 225) include twonegative edged flip-flops (210A, 210B and 230A, 230B) connected to eachother. Flip-flops 210A and 210B receive asynchronous clock signals CLK1(first clock signal) on line 250 and flip-flops 230A and 230B receiveCLK2 on lines 255 respectively. Outputs of the CLK2 enablesynchronization logic 230 (CLK2 enable on line 235) is connected to ANDgate 240. Output of the AND gate 240 (logic gate output) is connected toa priority multiplexer 220. An output of CLK1 enable synchronizer logic210 (CLK1 enable on line 215) and CLK1 on line 250 are also connected tothe priority multiplexer 220. An output from the priority multiplexer220 is the clock output CLK OUT, on line 260.

Operation of the clock switching circuit including enable generation andenable synchronization is same as that of clock switching circuitillustrated in FIG. 1 and as explained previously. In one embodiment,AND-OR logic (125, 130 and 140) structure on the clock switching circuit(FIG. 1) has been modified and is replaced with priority multiplexer(220). Also flip flops in CLK2 enable synchronization logic (230) arereset asynchronously using RESET signal. Due to the limitation aspreviously explained, AND gate (240) can either output CLK2 continuously(case 1) or outputs clock for 1 or two cycles (case 2). However due tothe priority multiplexer, clock on line 245 which is output of AND gate240, will not be propagated to CLKOUT (260) because during and afterreset (when RESET is high) CLK1 Enable (215) is high. When reset isreleased, CLK2 which is enabled (due to case2) for 1 or 2 cycle will notaffect the output of clock switching circuit. In other words, duringthis time the priority multiplexer (220) blocks the glitch propagatingto CLKOUT. In other words, the priority multiplexer 220 is configured toselect CLK1 as the clock output if CLK1 enable is a logic high,irrespective of the output of the AND gate 240.

In the event when one of the clock source is faulty, missing orinactive, switching the clock output to other clock is not possible dueto the existence of feedback from first clock domain (CLK1 155 connectedto enable generation logic 110) and from the second clock domain CLK2(CLK2 160 connected to enable generation logic 105). Solution to thislimitation is illustrated in FIG. 3. An embodiment includes a missingclock detection circuit 265 which can detect the absence of a clocksignal, which is common in fault tolerant ASICs. Outputs of the missingclock detection circuit 265 are CLK1 OFF on line 275 (indicative offaulty CLK1) and CLK2 OFF on line 270 (indicative of faulty CLK2). Inresponse to the CLK OFF signal (CLK1 OFF or CLK2OFF), a correspondingfeedback path is broken such that an output of the clock switchingcircuit is switched from one clock source to the other. In oneembodiment, CLK1 OFF and CLK2 OFF include two memory mapped bits whichcan be modified by the processor. By using these bits processorcommunicates missing clock information to the clock switching circuit.This information is used by the clock switching circuit to break thefeedback loop between CLK1 enable synchronization logic (210) and CLK2enable synchronization logic (230) and vice-versa.

Working of the clock switching circuit of FIG. 3 according to anembodiment is explained below. The clock switching circuit of FIG. 3includes a first feedback path (CLK2 enable coupled to enable generationlogic 205) and a second feedback path (CLK1 enable coupled to enablegeneration logic 225). Assume that CLK Select is low; which means clockswitching circuit is outputting CLK1 (250) on CLKOUT (260). Also assumethat during the operation, CLK1 (250) becomes faulty. Upon detectingCLK1 (250) fault condition, processor might switch to alternate clocksource (CLK2, 255) by changing the CLK select to low. However due to thefaulty clock (CLK1, 250) CLK1 Enable (215) will never become zerocausing a dead lock. This dead-lock/feedback can be broken bycommunicating faulty information to clock switching circuit usingCLK1OFF-M105, CLK2OFF-M205. In this case, CLK1 OFF (275) is made high toindicate that CLK1 (250) is faulty in addition to making the CLK selectto high. Then CLK1 Enable (215) is forced to low asynchronously hencebreaking the feedback loop. Since CLK1 enable is forced to low, D inputof CLK2 enable synchronization logic is made high immediately.Eventually this will make the CLK2 enable (235) high and CLK2 (255) willbe outputted on CLK OUT (260). Forcing CLK1 enable (215) to lowasynchronously does not cause any meta-stability issue on flop becauseCLK1 (250) is inactive.

In the foregoing discussion, the term “connected” means at least eithera direct electrical connection between the devices connected or anindirect connection through one or more passive or active intermediarydevices. The term “circuit” means at least either a single component ora multiplicity of components, either active or passive, that areconnected together to provide a desired function. The term “signal”means at least one current, voltage, charge, data, or other signal.

The forgoing description sets forth numerous specific details to conveya thorough understanding of the invention. However, it will be apparentto one skilled in the art that the invention may be practiced withoutthese specific details. Well-known features are sometimes not describedin detail in order to avoid obscuring the invention. Other variationsand embodiments are possible in light of above teachings, and it is thusintended that the scope of invention not be limited by this DetailedDescription, but only by the following Claims.

1. A clock switching circuit that provides a clock output selectablefrom a plurality of clock signals, the clock switching circuitcomprising: A. a first enable synchronization logic that generates afirst clock enable signal in response to a first enable signal from afirst enable generation logic, the first enable synchronization logicconfigured to receive a first clock signal; B. a second enablesynchronization logic that generates a second clock enable signal inresponse to a second enable signal from a second enable generationlogic, the second enable synchronization logic configured to receive asecond clock signal; C. a logic gate having one input connected to thesecond clock enable signal, another input connected to the second clocksignal, and an output; and D. a priority multiplexer having the clockoutput, a first input connected to the first clock signal, a secondinput connected to the logic gate output, and a control input connectedto the first clock enable signal.
 2. The clock switching circuit ofclaim 1, wherein at least the first clock signal and the second clocksignal of the plurality of clock signals are asynchronous clock signals.3. The clock switching circuit of claim 1, wherein the first enablesynchronization logic comprises a first plurality of negative edge flipflops that are coupled to each other and the second enablesynchronization logic comprises a second plurality of negative edgeflip-flops that are coupled to each other.
 4. The clock switchingcircuit of claim 3, wherein the first plurality of negative edge flipflops receives the first enable signal and generates the first clockenable signal; and wherein the second plurality of negative edge flipflops receives the second enable signal and generates the second clockenable signal.
 5. The clock switching circuit of claim 4, wherein theplurality of negative edge flip flops receives a RESET signal at powerup of the clock switching circuit.
 6. The clock switching circuit ofclaim 1, wherein the first enable generation logic generates the firstenable signal in response to a first clock select signal and the secondclock enable signal; and wherein the second enable generation logicgenerates the second enable signal in response to a second clock selectsignal and the second clock enable signal.
 7. The clock switchingcircuit of claim 1, wherein the first clock signal is a default clocksignal.
 8. The clock switching circuit of claim 7, wherein the firstenable signal is logic high at reset.
 9. The clock switching circuit ofclaim 1, wherein the priority multiplexer is configured to block aglitch at the logic gate output.
 10. The clock switching circuit ofclaim 1, wherein the logic gate includes an AND gate.